The massive growth in mobile broadband data traffic has driven the evolution of the cellular systems to meet the tremendous need for increased capacity. However, the traffic growth will be difficult to address with spectrum currently available for Long Term Evolution, LTE, systems, especially for the valuable bands with low propagation loss below 6 GHz. The licensed band is superior, from the quality and availability perspective, but scarce and expensive, especially below 6 GHz. Given the fact that the current amount of un-licensed spectrum is comparable to the amount of licensed spectrum, and the fact that in some regions, more un-licensed spectrum is planned to be allocated, it is natural for operators to look into the potential of utilizing un-licensed spectrum to meet the traffic growth demands.
In 3GPP (3rd Generation Partnership Project) studies on supporting LTE in un-licensed spectrum are likely to take place, focusing on low-power nodes. The basic set-up is “licensed-assisted access” with carrier aggregation, CA, between licensed and un-licensed carriers. A licensed LTE carrier is used for all mobility, control signalling and parts of the user data, while one or more carriers in un-licensed spectrum are used to boost the user-data performance.
The basic IEEE 802.11 MAC employs a Carrier Sense Multiple Access with Collision Avoidance based MAC. The same protocol is applied by all stations, including the access point, i.e. in both downlink and uplink. A station that wishes to transmit a packet first senses the medium. If the medium is sensed idle for a certain minimum time, a so-called Distributed Inter Frame Space, DIFS, (50 μs for IEEE 802.11b), the packet is transmitted. If the medium is busy, the station first defers until the medium is sensed idle. Hence that different transmission points will compete to transmit on the shared channel and a package data unit is not transmitted in predefined time point. Since base stations e.g. evolved NodeBs (eNBs) in LTE systems do not currently have the capability of performing measurement on the un-licensed frequency band, the fairness between different systems operating in the same un-licensed frequency band will not be guaranteed.